The invention relates to a process for hardening workpieces of steel, especially those having at least one alloy element from the group Cr, Ni, Mn, Si and Mo by carburizing the surface and then quenching. The carburizing is performed by a plasma discharge in a vacuum in the presence of gaseous hydrocarbons at partial pressures between 1 and 20 mbar (100 to 2000 Pa) and at voltages between 200 and 2000 volts, preferably between 300 and 1000 volts. The plasma is produced by means of electrodes operated in a vacuum, the cathode serving as workpiece holder and being operated in pulsed mode.
U.S. Pat. No. 4,881,982 discloses that if the rate of delivery (mass flow m.sub.c) of the carbon to the workpiece surface is too high, carbon supersaturation occurs which results in the formation of carbides. This drastically lowers the high hardness already achieved by the carburization. Ideally, the carbon content and hardness characteristic should be able to be represented in a diagram as approximately S-shaped curves.
Reducing the supersaturation by diffusing carbon into the depth of the workpiece would only be possible by extremely lengthy diffusion processes. It is therefore proposed in the literature to cycle back and forth repeatedly between the carburizing phase and the diffusion phase so as to give the carbon the chance to diffuse to the necessary depth in the workpiece. The cycle times are long and the determination of the time at which to cycle is difficult and therefore inaccurate.
U.S. Pat. No. 4,900,371 discloses a pulsed plasma process of the kind described above in which the repetition time is 10 milliseconds and the pulse and pause times amount each to 5 ms. The parameters stated are said to result in making the gas and plasma distribution over the workpiece surface uniform, but at the usual cathode voltages of 500 to 1000 V, mass flows of carbon result, which without the insertion of carburization-free diffusion phases, would within a few minutes lead to the supersaturation of the surface area with carbon and thus to the undesired formation of carbide. From the data given, a mass flow of carbon results, of EQU m.sub.c =approx.80 g m.sup.-2 h.sup.-1
A further problem lies in the fact that in plasma carburization the process is performed in the range of the so-called anomalous incandescent discharge, in which, in the event of an increase of the voltage from about 200 to over 1000 volts, the current density increases disproportionately until the anomalous discharge abruptly changes to an arc discharge after a limit voltage is exceeded (see: (1) Bell/Loh/Staines "Thermochemische Behandlung im Plasma," NEUE HUTTE, vol 28, No. 10, October 1983, pages 373 to 379; (2) Booth/Farrell/Johnson, "The Theory and Practice of Plasma Carburizing" HEAT TREATMENT OF METALS, 1983, pp. 45 to 52).
This process is to be avoided under all circumstances, since it would result in damage to the workpiece. Under the conditions of U.S. Pat. No. 4,900,371 a shift from the anomalous incandescent discharge to an arc discharge cannot be excluded with sufficient reliability.
U.S. Pat. No. 4,490,190, which corresponds to EP-B 062 550, discloses a process operated at constant power throughout its duration in which a pulse duration very much shorter than the duration of the period is selected in order to make two treatment parameters independent of one another, namely the plasma on the one hand and the treatment temperature on the other. This problem, however, exists only in the case of nitridation and carbonitridation, since the treatment temperatures in this case must be definitely below 600.degree. C. Under the stated conditions, carburization is not possible within economically acceptable treatment times, since this process does not take place at a practical rate until the temperatures are above about 800.degree. C.